Added Media.AddChatLine to the new Lua API

Unmerged part of https://github.com/OpenRA/OpenRA/pull/5469.
I wonder why you all have so much trouble to understand it. It is a simplified wrapper of `Game.AddChatLine` which was used all the time in https://github.com/OpenRA/OpenRA/tree/release-20131223/OpenRA.Mods.RA/Missions. I don't think renaming it Print or even hiding the system Lua print is a good idea. Will look into adding more parameters though.